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Model S regen

Discussion in 'Model S: Driving Dynamics' started by Doug_G, Oct 14, 2011.

  1. Doug_G

    Doug_G Lead Moderator

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    I don't think this is by intent; rather, it is a limitation of how fast the batteries can swallow regen energy. The car is significantly heavier.
     
  2. doug

    doug Administrator / Head Moderator

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    Hmm.. well how fast can the batteries take energy? We know they should at least be able to do 90 kW.

    Back of the envelope, if the car weighs 4000 lbs, 60 to 0 mph purely on 90 kW regen would take 7.3 seconds.

    Tesla says out of the motor they get 360 hp =~ 270 kW, so if you got that full regen, you'd get 2.4 seconds. (seems plenty quick)

    The gauge on the Roadster suggested its regen was limited to 40kW.
     
  3. TEG

    TEG TMC Moderator

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    I suspect that there are complexities to the way regen works that limit it for other reasons.

    A quickie search turned up this possibly relevant example? (although maybe not since it describes DC motor regen) :
    http://www.cafeelectric.com/curtis/regen/index.html

    Anyways, having the motor run in regen mode may not be as simple as reversing the capability of forward-drive mode. There may be some additional circuits or constraints that limit the max regen power as compared to the max power the inverter & motor can make when accelerating.


     
  4. TEG

    TEG TMC Moderator

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    The Australian Electric Vehicle Asn: cntrolling regen
     
  5. rabar10

    rabar10 FFE until Model 3

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    Another question is at what point regen becomes torque-limited (i.e. motor-current-limited) as opposed to power-limited (i.e battery-current-limited).

    The current Model S torque graph shows flat torque up to ~57mph, where the peak power point is, and then power gradually falls off to ~80% of peak up to ~120mph.

    Under regen, I don't know what the maximum negative torque would be and at what MPH that would be reached, but I know regen in the Roadster gets weaker the slower the car gets, so it has to hit that negative torque ceiling somewhere. (Part of the weakening Roadster regen at very slow speeds would also be due to allowing creep, i.e. it won't slow the vehicle to a full stop so it approaches the creep speed.)

    Separate question, but one that could enlighten this conversation on Model S regen strength: has anyone produced a regen / negative torque graph from their Roadster?
     
  6. doug

    doug Administrator / Head Moderator

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    Good point.
     
  7. rabar10

    rabar10 FFE until Model 3

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    (thanks for splitting the thread)

    Wanted to ask this again: has anyone produced a regen / negative torque graph from their Roadster log data? It might provide insight on what to expect from Model S...
     
  8. neroden

    neroden Happy Model S Owner

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    Hmm. I hope it's not by intent; I like the "minimum-brake-pedal driving" thing and used to do it all the time in manual-transmission cars at low speed.

    But I guess a heavier car would definitely have more momentum, so you'd have to feed more energy as regen in order to get the same level of "motor braking". But anyway I would hope the settings include the maximum viable level of regen.
     
  9. JRP3

    JRP3 Hyperactive Member

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    No such complexities exist in AC induction, the limits are all heat and battery related, probably mostly battery. Most Li cells have higher discharge than charge rates, and putting too much current into them too quickly drives the voltage above the breakdown voltage of the electrolyte, which is around 4.5V I think.
     
  10. Thumper

    Thumper Member

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    Have I conceptualized this wrong? Isn't regen limited to exactly the charger capacity. I think I have seen 10KW on the boards. It is the same problem for the car to turn AC into DC whether from the AC motor or the wall. Nicht wahr?
     
  11. JRP3

    JRP3 Hyperactive Member

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    I don't think Tesla uses the motor windings and inverter as part of the charger the way ACP used to, so no there should be no connection between regen capability and charging capability. Even so charging is limited to power from the wall where regen is not.
    Theoretically the motor should be able to regen back as much power as it can take going into it for acceleration.
     
  12. Tommy

    Tommy Member

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    The limit Tesla has put on the regen may be artificial. The driving experience for model S needs to mimic a sport sedan not a roadster. From what I've read, the roadster decelerates very fast, akin to moderate/hard braking when in full regen mode. Fun in a roadster, but not the sensation one expects in a luxury sport sedan. As an example, there are times when I drive that I take my foot off of the "accelerator" just to find a more comfortable position; to have the car suddenly decelerate like the roadster would be very disconcerting. Tesla has to take into account the driving expectations, and not just how much regen is possible for the battery size, weight of car, available traction, etc.
     
  13. Thumper

    Thumper Member

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    Thanks, I had misunderstood.
     
  14. strider

    strider Active Member

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    All the more reason to make it a user preference. Leave it stock at a "light" level for the Bimmer and Merc drivers and as they come to like the one-pedal driving and for those of us who are used to driving EVs, let us crank it up. My wife only drives the Roadster a couple times per month but she adapts within a couple stop signs every time. When I let friends drive my car they love the regen.
     
  15. Kevin Harney

    Kevin Harney Active Member

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    My BMW has LOTS of compression. I love it. I drove the Roadster as well and loved the regen. The more of it the better for me. I do not know if Mercedes has much less compression and a longer coast or not ....
     
  16. Andrew Wolfe

    Andrew Wolfe Roadster 472 - S 440

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    It depends if everything is being put back into the battery. There have been a number of designs shown over the years, both for Electric and Hybrid, where regen is stored short term in Ultracapacitors. They have an almost unlimited charge rate. Honda pushed this for a while. I'm not sure if it ever made it into production. All the EV companies know how to do this - but there are cost/complexity issues. Not clear if this is part of the Model S or not.
     
  17. Doug_G

    Doug_G Lead Moderator

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    If the regen is that light, I will be disappointed. The regen is one of the best parts of the Roadster driving experience.
     
  18. WhiteKnight

    WhiteKnight _____ P85 #549 _____ Sig Red / Sig White

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    According to reports the Regen will be user-selectable via the touchscreen:

    Tesla Model S Customer Blog: Beta Under The Bright Lights | The Truth About Cars
     
  19. doug

    doug Administrator / Head Moderator

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    Well, max possible regen is limited by the batteries and the motor, so should be close to max acceleration. So say 60 to 0 in 5.6 seconds.
     
  20. JRP3

    JRP3 Hyperactive Member

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    Depends on the charge/discharge symmetry of the cells. I don't know about the LiCo or the new LiNiCoAl cells but most of the LiFePO4 cells have higher discharge rates than charge rates.
     

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